What are Probiotics?

The term “probiotic” (from the Greek for life) has had a number of different meanings over recent history. First coined by Lilley and Stillwell, they used the term to describe substances secreted by one microorganism that stimulated the growth of another.[1] In 1974, this definition was modified by Parker to mean “…organisms and substances which contribute to intestinal microbial balance”, which more closely approximates the contemporary use of the term.[2] The current definition, comes from the International Scientific Association for Probiotics and Prebiotics.[3] They define probiotics as “live microorganisms that, when administered in adequate amounts, confer a health benefit to the host”.

The current definition of probiotics includes preparations that contain viable, microbial agents that have been demonstrated to improve health. Typically, these products will contain freeze-dried (lyophilized) or live bacteria or yeasts; most commonly from the genera Lactobacillus and Bifidobacterium.[4] Probiotic organisms can be incorporated into supplements (powders, capsules, tablets, oils, wafers), as well as foods (milk drinks, medicinal yoghurts, fruit juices, confectionery bars, ice cream). All of these mediums essentially work as carriers for the probiotic organisms.

What are not probiotics?
The early definitions of probiotics were inclusive of traditional fermented foods such as yoghurt (non-medicinal varieties), sauerkraut, and kefir, but the most recent interpretation of the definition has, somewhat controversially, excluded these traditional ferments. These are now considered food sources of “live and active cultures”, but not probiotics.[3]

The reasoning behind this decision is that these foods are of undefined microbial content. Species and strain composition can differ from batch to batch, as can bacterial counts. Additionally, the strains contained in these foods may also lack specific therapeutic qualities – e.g., they may not confer any health benefit on the host, beyond the enhanced nutritional profile of the fermented food. It is for these reasons that traditional fermented foods (wild ferments) cannot be relied upon for specific therapeutic effects in the same way that probiotic preparations containing well-defined strains, with well-characterized clinical effects, in precise doses can.


1. Lilley, D.M. and R.H. Stillwell, Probiotics: growth promoting factors produced by microorganisms. Science, 1965. 147: p. 747-748.
2. Parker, R.B., Probiotics, the other half of the antibiotic story. Animal Nut Hlth, 1974. 29: p. 4-8.
3. Hill, C., et al., Expert consensus document: The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nat Rev Gastroenterol Hepatol, 2014. 11(8): p. 506-514.
4. Collins, M.D. and G.R. Gibson, Probiotics, prebiotics, and synbiotics: approaches for modulating the microbial ecology of the gut. American Journal of Clinical Nutriton, 1999. 69((suppl)): p. 1052S-1057S.